A commonly used technique for launching a missile from a submerged submarine involves admitting sufficient pressurized gas (hereinafter "launching gas") into the submarine launch tube containing the missile to both overcome the static water pressure head surrounding the submarine and to propel the missile upward with such a velocity as to cause the missile to breach the surface of the sea and to travel beyond up into the atmosphere, after which the missile rocket motor ignites. The missile body displaces water as it travels upward toward the surface. As the missile is slowing down as it rises, water gushing into the space left behind can produce a spout of water having a speed greater than that of the upwardly moving missile, which can spurt upward and damage the missile rocket motor after the missile itself has breached the surface of the sea.
Solid-propellent-powered rocket motors contain internal voids, intentionally designed so as to form a desired burning surface pattern within the solid propellant grain. These internal voids must be vented in order to prevent collapsing of the rocket motor case due to the external gas pressure of the launch environment. A gas tight seal over the nozzle would prevent the launching gas from equalizing pressure inside the rocket motor and would thereby endanger the structural integrity of the missile. The surrounding external water pressure decreases as the missile travels upward toward the surface of the sea; as a consequence the compressed launching gas (which was contained within the rocket motor interior at launch) expands and streams out through the rocket motor nozzle underwater. Any protection system must permit the flow of launching gas both into and out of the rocket motor, while protecting the rocket motor against an upwardly spurting spout of water.
Nozzle throat baffles made of disks of reticulated aluminum (one placed on each side of a nozzle throat, and the two tied together by a connecting tension element) have been used to exclude solid foreign material from entering solid propellant rocket motors, but they are not too effective against fluids as such baffles functionally are merely three-dimensional metal-wire screens. Reticulated aluminum is an open-cell foam of aluminum metal. Baffles of reticulated aluminum can be designed so that under sufficient gas pressure (e.g., the exhaust from an ignited rocket motor) the forward baffle will deform and both will be expelled from the nozzle. Existing expellable baffles can, however, be utilized as an attachment point for other more effective water exclusion devices.
Some prior designs of rocket motor water exclusion devices are gas bag designs that include attachment to the aft rim of the rocket motor nozzle, and a rigid shield design that protrudes beyond the aft rim of the rocket motor nozzle. Any fixed device design (such as a rigid shield) that protrudes beyond the aft rim of a missile rocket motor nozzle imposses a missile body length reduction penalty on missiles which must fit within existing submarine launch tubes. Any gas bag design which includes attachment to the nozzle rim effectively seals off the rocket nozzle, and thereby imposes a requirement for valving to accommodate the passage of launching gas to and from the interior of the rocket motor grain. Additionally break-away means are required to allow the bag to release from the nozzle rim after the rocket motor has ignited.
Therefore a need exists for a simple and reliable rocket motor water exclusion device that will permit the flow of launching gas both into and out of the rocket motor grain interior, that will take up minimal space within the nozzle, and that will be reliably expelled from the nozzle upon ignition of the rocket motor.